1. Field of the Invention
The present invention relates to a battery pack and a battery pack producing method.
2. Description of Related Art
Recently, various secondary batteries have been proposed as the power source for portable devices and mobile devices or as the power source for electric vehicles or hybrid electric vehicles. When the secondary batteries are used as the power source for electric vehicles or hybrid electric vehicles, high output power is demanded and hence a plurality of secondary batteries are electrically connected in series to be used as a battery pack.
However, in the battery pack used as the power source for electric vehicles or hybrid electric vehicles, one or more of the secondary batteries constituting the battery pack are often broken earlier than other batteries due to environments of use, characteristic difference among individual secondary batteries of the battery pack, or troubles of component parts of each secondary battery. As a result, the intended performance of the battery pack may not be exhibited, which may cause the entire system failure. To solve such problems, a secondary battery broken earlier than other batteries must be replaced with a normal secondary battery. Various methods have been proposed for replacing the secondary batteries (see, for example, JP2004-185915A).
JP2004-185915A discloses a method of replacing secondary batteries of a battery pack in which a plurality of secondary batteries are electrically connected in series or in parallel. According to this method, when a certain defective secondary battery is to be replaced with a new secondary battery for exchange, a charge amount of the new secondary battery is controlled to be smaller than charge amounts of other batteries not to be replaced (normal secondary batteries of the battery pack). Specifically, the new secondary battery is charged so that the charge amount thereof may be 5% to 20% smaller than the charge amounts of other normal batteries not to be replaced. As a result, according to JP'915A, when charging and discharging are repeated during use of the battery pack, the difference in charge amount (the charged electric quantity) between the new secondary battery and other existing batteries becomes smaller, and hence the charge amounts of secondary batteries of the battery pack become equal. It is also said that the performance of the battery pack can be exhibited to a maximum extent.
Even when electric vehicles or hybrid electric vehicles are to be deregistered or scrapped, the battery packs mounted thereon may still be in a usable state. Further, even in the case where the battery pack is replaced with a new battery pack due to defective one(s) of the secondary batteries of the battery pack, the other secondary batteries in the discarded battery pack may still be in a usable state. A new technique is therefore demanded for re-using the usable secondary batteries without discarding, that is, usable secondary batteries among secondary batteries used and collected from the market (hereinafter, also referred to as “used secondary battery(s)”). For example, there is demand for a technique of producing a new battery pack by collecting used battery packs from scrapped vehicles or the like and combining usable secondary batteries (used secondary batteries) contained in those collected used battery packs.
However, the collected used secondary batteries contain those used in various environments and thus battery characteristics are often largely different from battery to battery. Such difference in battery characteristics may cause some problems when a battery pack is produced by combining the used secondary batteries. For example, one or more of the used secondary batteries constituting the battery pack may be overcharged or overdischarged and the performance of the used secondary batteries of the battery pack may not be exhibited sufficiently. JP'915A and others disclose various techniques for replacing one or more of secondary batteries constituting the battery pack (defective secondary batteries) with new and normal secondary batteries. However, such techniques could not be applied in the technique of producing a new battery pack by combining used secondary batteries.